1. Field of the Invention
The present invention relates to a solder powder which comprises solder particles having a protective layer formed thereon composed of a metal salt of adipic acid, a method for making the solder powder and a solder paste using the solder powder.
2. Description of the Prior Art
With the recent wide-spread use of compact, light-weight apparatuses, such as a portable telephone, the electronic circuits which are to be incorporated in these apparatus are being miniaturized and more densely packed, highlighting the increasing importance of the technology of mounting a variety of parts on a circuit substrate. Particularly, the technical level of micro-soldering, which is the most important mounting technology, is becoming increasingly higher. As a result, a strict technical requirement is placed on the solder paste. For example, the particle size of the solder powder is becoming smaller and yet the generation of solder balls must be minimized because of the adoption of a manufacturing process which abolishes a cleaning treatment of a soldered product due to the phasing-out of flons, i.e., chlorofluorocarbons. The solder balls are generated in the soldering operation mainly due to the presence of an oxide film on the surface of the particles of the solder powder. Since this oxide film does not break off even if the interior of the solder particles are fused when the temperature reaches the melting point of the solder, the solder particles cannot combine with other fused solder portion and thus small balls are formed. This phenomenon becomes particularly conspicuous when a fine solder powder is used in order to enhance the printability. Based on the above-described background, many attempts have been made to inhibit the surface oxidation to thereby inhibit the generation of the solder balls. Examples of these attempts include a technique wherein a solder powder is produced in an atmosphere of an inert gas, a technique wherein the solder particle is made as spherical as possible to decrease the surface area and a technique wherein fine solder particles which are more susceptible to the oxidation are eliminated. These techniques, however, have drawbacks, for example, that the inhibition of the solder ball generation by these techniques is limited and that the surface of the solder particles is damaged when the fine particles are eliminated by means of sieving. Moreover, the elimination of fine solder particles by sieving is disadvantageous from the viewpoint of the current trend of using fine particles for finer pattern printing. Anyway, an attempt to decrease the oxidation of the surface of the solder particle itself by the above-mentioned techniques is not successful because it is impossible to provide an oxygen-free atmosphere for all steps, such as storing or handling of the solder powder, making a paste from the solder powder, printing, transferring to a reflow oven and reflow soldering, even if the solder powder is produced in a strictly controlled, oxygen-free atmosphere. Consequently, the oxidation of the solder powder at the above-mentioned steps cannot be perfectly prevented. Yet another method for minimizing the generation of solder balls by preventing the oxidation of the solder powder at the above-mentioned steps includes the incorporation of an active ingredient in a slightly excessive amount for the removal of the oxide in the flux at the time of impasting the solder powder. The resulting residual flux after soldering is removed by cleaning with a flon. However, according to the scheduled phase-out of flons in view of environmental protection, a manufacturing process which dispenses with the cleaning step by reducing the incorporated amount of the active ingredient is intensively studied in order to increase the reliability of the products.
For example, U.S. Pat. No. 4,298,407 proposes a tin-based alloy solder powder composition which comprises particles of a tin-based alloy solder coated with a flux composed of an organic acid. The types of the coating flux according to this U.S. patent include a dicarboxylic acid, more particularly, succinic acid and salicylic acid. According to the U.S. patent, since the organic acid coated onto the alloy particles dissolves the tin oxide formed on the surface of the solder particles and thus cleans the surface of the tin-containing solder particles, the generation of the solder balls is inhibited. Such an organic acid is blended with the solder powder by means of a beads mill for a short period of time so that the particles of the solder powder are coated with the organic acid.
One problem of the invention of the above-mentioned U.S. patent is that, since the organic acid layer is formed by mere mechanical blending by means of the beads mill, the organic acid merely adheres to the surface of the solder alloy particles, so that the adhesive strength of the organic acid to the particles of the solder powder is weak and the organic acid partly peels off the particles at the time of handling or impasting the solder powder and, therefore, the oxidation prevention of the solder powder is insufficient. Another problem is that, when a paste is produced from this solder powder, the organic solvent contained in the paste dissolves away the organic acid to remove the organic acid layer from the particles of the solder powder and that the oxidation of the solder powder is accelerated at a reflow step because of the presence of the damage which has been caused by the treatment with beads on the surface of the particles of the solder powder. In addition, it is an art-recognized fact that the above-mentioned organic acids are not so effective with lead oxide in the case of a lead-containing solder alloy, which is commonly used as a solder alloy, although these organic acids are effective in dissolving the tin oxide to clean the solder particle surface. Therefore, the use of a flux which contains a substance such as isopropylamine or triethanolamine, is considered indispensable for inhibiting the generation of solder balls in the case of a lead-containing solder alloy.